Add the device tree binding documentation for the ov7740 sensor driver.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>